Known in the art is a device for measuring average electric conductivity of plasma (cf. USSR Inventor's Certificate No. 577477, Cl. G 01 R 27/22, publ. in 1977), comprising an inductive sensor connected in a measuring circuit of a high-frequency oscillator and having a measuring coil and a compensation coil which are wound in a bifilar fashion, a detector, a recorder and a logical switching circuit connected to said coils through controlled switches.
In this device the inductive sensor of the straight way type is connected in the circuit of a high-frequency oscillator having a working frequency of about 200 kHz. The sensitive element of the sensor has two electrically identical bifilar coils of which one coil is a measuring coil and the other coil is a compensation coil. These coils are connected, via switches controlled by a logical switching circuit, to said high-frequency oscillator. The windings of the measuring coil are connected through one of the switches in opposition to one another and the windings of the compensation coil are concordantly connected through the other switch as one alternative. The sequence of switching of the bifilar windings in the prior art circuit consists in a cyclic switching of said controlled switches.
The disadvantages of the prior art device reside in the absence of possibility of measurement of local electric conductivity of plasma in local zones of about 1 to 10 mm and relatively low accuracy of measurements of average electric conductivity of plasma owing to instability of the high-frequency oscillator due to a non-linear variation of reactive component of the resistance of the windings of measuring circuit at working frequencies of hundreds kHz and at a temperature above 100.degree. C.
Known in the art is also a device for measuring local electric conductivity of plasma (cf. N. A. Balashov et al., Studies of Temperature Dependence of Electric Conductivity of Working Fluid of Large MND Generators (in Russian), Teplofizika vysokikh temperatur, vol. 15, No. 6, 1977, p. 188). The device comprises a movable probe in the form of a wing-shaped rod made of a non-conductive material having at the end thereof an inductive sensor. The sensor is coupled, via a measuring oscillator and a detector, to a unit for recording measurements via a differential stage. The measuring oscillator, detector, differential stage and unit for recording measurements are arranged outside the movable probe. The probe is caused to move by means of a pneumatic probe drive mechanism. The calibration of the inductive sensor is effected during movement of the probe through the calibration unit made as an electrically conductive ring mounted at the end face of the pneumatic probe drive mechanism.
The probe movement is controlled by means of air valves of an electropneumatic control unit which are installed directly on the probe drive mechanism.
When high-frequency oscillations are generated in the coil of the inductive sensor, eddy currents are induced in the electrically conductive medium which cause a reduction of the Q factor of resonant oscillator circuit proportionally with the electric conductivity of the medium thus resulting in a decrease in the amplitude of self-oscillations of the measuring oscillator. A signal proportional with the amplitude of the measuring oscillator is detected in the control unit and is fed, via the differential stage, to the unit for recording measurements.
The disadvantage of the prior art device resides in that it is not possible to measure electric conductivity of plasma in the presence of strong magnetic fields (hundreds Gausses) which are characteristic of MHD generators, owing to possible misfunctions of electropneumatic elements of the probe drive system and some other systems of the device, such as the unit for recording measurements.
Such device has inadequate sensitivity and high error value (more than 5%) in measuring electric conductivity of plasma which is due to a limitation of maximum frequency of the measuring oscillator because of the need to arrange the electropneumatic control unit and measuring oscillator outside the probe.
The prior art device cannot have a local zone in which the electric conductivity of plasma is measured smaller than 100 mm which is not sufficient for an exact determination of the pattern of the electric conductivity, in particular, in the electrode zone of the channel of an MHD generator.
In addition, a Hall effect EMF developed in operation of an MHD generator does not make it possible to measure local electric conductivity owing to the absence of a high-potential uncoupling of measuring oscillator and recording unit.
Service life of the probe in measuring electric conductivity of plasma in the range of 2-30 S/m at a temperature up to 3200 K., e.g. in the combustion chamber of an MHD generator is very short and is about 20 measurements.